IEEE 802.11ac provides an arrangement for channel estimation of the multiple downlink channels from an AP to multiple STAs. The channel estimation involves a process called sounding. Various schemes can be used to perform sounding. For example, an AP can transmit a Null Data Packet-Announcement (NDP-A). The NDP-A contains the addresses of particular STAs to which the AP wishes to transmit data. The NDP-A is followed by a Null Data Packet (NDP). The NDP represents pilot energy; that is, a pre-defined pattern. Each STA addressed by the NDP-A measures the downlink wireless channel from the AP to itself. The channel measurement is possible because the given STA knows the pre-defined pattern and can estimate the effects of the channel on that pattern (for example, amplitude attenuation, carrier phase rotation, and time delay). The various STAs feedback their channel measurements, and then the AP has knowledge of the wireless channels from the AP to the STAs.
However, NDP-A and NDP messages and similar sounding management and pilot messages create an overhead. That is, these sounding-specific messages consume both bandwidth and energy. There is a tradeoff between placing energy into pilot signals and placing that energy into medium access control (MAC) data frames. For a shared medium every transmission of energy is potentially communication-limiting to some other unrelated simultaneous transmission.